Pneumatic tire

ABSTRACT

A pneumatic tire includes circumferential grooves aligned in a lateral direction and extending in a circumferential direction, lateral grooves aligned in the circumferential direction and extending to cross the circumferential direction, communicating at ends with the circumferential grooves and defining a block-shaped land portion between the circumferential grooves, two narrow grooves aligned in the lateral direction and extending in the circumferential direction, communicating at ends with a lateral groove and dividing the land portion defined by each of the circumferential grooves and each of the lateral grooves into small land portions, the narrow grooves having a smaller groove width than the circumferential grooves and being formed with a bent portion in an intermediate portion, the bent portion being disposed inward in the lateral direction, and bend points of the bent portions being disposed at positions offset from one another in the circumferential direction.

TECHNICAL FIELD

The present technology relates to a pneumatic tire.

BACKGROUND ART

In a typical tire, uneven wear is caused by high amounts of pressureacting on the tire when it comes into contact with the ground. Thus,ground contact pressure needs to be appropriately alleviated.

A known pneumatic tire, for example the pneumatic tire described inJapan Patent Publication No. 2003-182315, is designed to improvesteering stability on snow in a travel-worn stage without decreasing drysteering stability. This pneumatic tire includes, in a road contactsurface portion of a tread portion, a plurality of main groovesextending in a circumferential direction, and a block defined by lateralgrooves that cross the main grooves. The block includes sipes and a pairof acute angle block corner portions on opposite sides in thecircumferential direction. The blocks further include a pair of smallblocks defined by the pair of acute angle block corner portions and twoof the sipes and a central block disposed between the small blocks, thecentral block being larger than the small blocks.

The pneumatic tire described in Japan Patent Publication No.2008-260343, for example, includes, in a belt layer, a small-angle beltincluding cords inclined at an angle ranging from 5° to 30° with respectto a tire circumferential direction and a large-angle belt includingcords inclined at an angle ranging from 45° to 90° with respect to thetire circumferential direction.

The pneumatic tire described in Japan Patent Publication No.2015-174469, for another example, includes a belt layer with amultilayer structure including a pair of cross belt layers and acircumferential belt.

The rigidity of blocks and ribs is reduced by narrow grooves and slitssuch as sipes being formed. This may lead to separation and chipping.

SUMMARY

The present technology provides a pneumatic tire that can provideimproved uneven wear resistance performance while maintaining durabilityperformance by alleviating ground contact pressure without reducingblock rigidity.

A heavy duty pneumatic tire mountable on a truck or bus, especially apneumatic tire with a low aspect ratio, may include a small-angle beltsuch as that in Japan Patent Publication No. 2008-260343 or acircumferential belt such as that in Japan Patent Publication No.2015-174469 (referred to below collectively as “circumferential belt” toretain the shape of the tread portion.

In the tire equatorial plane region where the circumferential belt isdisposed, the circumferential belt provides high circumferentialrigidity, thus allowing radial growth to be suppressed when the tire isnew and thereafter. However, the regions outward from thecircumferential belt in the tire lateral direction experience highradial growth due to circumferential rigidity being relatively lowcompared to that of the region at or near the tire equatorial plane.This causes uneven wear.

In light of the foregoing, the present technology provides a pneumatictire that can provide improved uneven wear resistance performance evenwith a configuration including a circumferential belt.

A pneumatic tire according to an aspect of the present technologyincludes two circumferential grooves, in a tread surface of a treadportion, disposed in alignment in a tire lateral direction extending ina tire circumferential direction, a plurality of lateral grooves, in thetread surface, disposed in alignment in the tire circumferentialdirection extending to cross the tire circumferential direction, theplurality of lateral grooves communicating at both ends with both of thecircumferential grooves and defining a block-shaped land portion eachbetween the circumferential grooves, a narrow groove, in the treadsurface of the land portion, disposed in alignment in the tire lateraldirection extending in the tire circumferential direction, the narrowgroove communicating at both ends with a respective lateral groove ofthe plurality of lateral grooves and dividing, in the tire lateraldirection, the land portion defined by each of the circumferentialgrooves and each of the lateral grooves into a plurality of small landportions, wherein the two narrow grooves are formed in the land portionhaving a smaller groove width than the circumferential grooves and eachincluding a bent portion in an intermediate portion, the bent portionbeing disposed inward in the tire lateral direction in a direction thenarrow grooves oppose one another from an imaginary straight lineconnecting ends of the respective narrow grooves, and bend points of thebent portions being disposed at positions offset from one another in thetire circumferential direction.

According to the pneumatic tire, each of the land portions defined bythe circumferential grooves and the lateral grooves are divided by thetwo narrow grooves to form small land portions. This results in lowerrigidity, which allows the ground contact pressure when the treadportion comes into contact with the ground to be alleviated.Additionally, the length of the narrow grooves is increased by the bentportions formed in the narrow grooves. This further results in lowerrigidity, which allows the ground contact pressure to be furtheralleviated. Thus, uneven wear resistance performance can be improved.Furthermore, according to the pneumatic tire, the bent portion isdisposed inward in the tire lateral direction in the direction the twonarrow grooves oppose one another from the imaginary straight lineconnecting the ends of the narrow groove. As a result, a reduction inthe area of the side small land portions on the tread surface issuppressed, and a decrease in rigidity is prevented. Moreover, the bendpoints of the two narrow grooves are located offset from each other inthe tire circumferential direction so that localized narrowing of thecentral small land portion is suppressed, and thus, a decrease inrigidity is prevented. Thus, separation and chipping of the land portioncan be suppressed. As a result, according to the pneumatic tire, unevenwear resistance performance can be improved while maintaining durabilityperformance by suppressing separation and chipping.

In the pneumatic tire according to an aspect of the present technology,preferably the bent portion of the narrow groove has a bend angleranging from 90° to 160°.

According to the pneumatic tire, when the bend angle of the bent portionis 90° or greater, the bend becomes less sharp. As a result, separationand chipping is less likely to occur and the effect of maintainingdurability is great. When the bending angle of the bent portion is 160°or less, the length of the narrow grooves is increased, and the effectof reducing rigidity is great.

In the pneumatic tire according to an aspect of the present technology,preferably a relative shift width Lc in the tire circumferentialdirection of each of the bend point of the two narrow grooves and a tirecircumferential direction dimension L of the land portion in which thenarrow grooves are formed satisfy 0.1≤Lc/L.

According to the pneumatic tire, by satisfying 0.1≤Lc/L, localizednarrowing of the central small land portion between the two narrowgrooves is further suppressed, and a decrease in rigidity can be furtherprevented. As a result, the effect of maintaining durability performanceis great.

In the pneumatic tire according to an aspect of the present technology,preferably the lateral groove is formed extending at an incline withrespect to the tire lateral direction, with an angle with respect to thetire lateral direction ranging from 5° to 50°.

According to the pneumatic tire, when the angle of the lateral groovewith respect to the tire lateral direction is greater than 5°, thelength of the lateral groove is increased and the effect of reducingrigidity is great. When the angle of the lateral groove is 50° or less,the angle is prevented from being sharp. As a result, separation andchipping is less likely to occur and the effect of maintainingdurability is great.

In the pneumatic tire according to an aspect of the present technology,preferably relationships Ha>Hb, and Ha>Hc are satisfied, where Ha is agroove depth of the circumferential grooves, Hb is a groove depth of thelateral grooves, and Hc is a groove depth of the narrow grooves.

According to the pneumatic tire, the groove depth Hb of the lateralgrooves and the groove depth Hc of the narrow grooves are less than thegroove depth Ha of the circumferential grooves so that a decrease inrigidity of the land portion in the tire circumferential direction issuppressed. Thus, the effect of maintaining durability is great.

In the pneumatic tire according to an aspect of the present technology,preferably Hb ranges from 1 mm to 5 mm and Hc ranges from 1 mm to 5 mm,where Hb is a groove depth of the lateral grooves and Hc is a groovedepth of the narrow grooves.

According to the pneumatic tire, the groove depth Hb of the lateralgrooves and the groove depth Hc of the narrow grooves range from 1 mm to5 mm to suppress a decrease in rigidity of the land portions. As aresult, the effect of maintaining durability is great.

In the pneumatic tire according to an aspect of the present technology,preferably the lateral grooves are formed extending at an incline withrespect to the tire lateral direction; and a chamfer is formed in acorner portion of the land portion with an acute angle with respect tothe tire circumferential direction.

According to the pneumatic tire, by providing the chamfer, separationand chipping are less likely to occur and the effect of maintainingdurability is great.

The pneumatic tire according to an aspect of the present technologypreferably further includes a sipe, in the tread surface, communicatingat one end with one of the circumferential grooves and terminating atanother end within the land portion; and preferably relationships 0.3mm≤Wd≤2.0 mm, 0.3≤Hd/Ha≤1.0, and 0.03≤Ld/We≤0.2 are satisfied, where Wdis a groove width of the sipe, Hd is a groove depth of the sipe, Ld is agroove length of the sipe, We is a tire lateral direction dimension ofthe land portion, and Ha is a groove depth of the circumferentialgrooves.

According to the pneumatic tire, the sipe reduces the rigidity of theland portion so that ground contact pressure is alleviated. As a result,the effect of improving uneven wear resistance performance is great.

A pneumatic tire according to an aspect of the present technologyincludes a circumferential belt, in a tread portion, disposed in a tirelateral direction encompassing a position of a tire equatorial planeincluding cords extending in a tire circumferential direction disposedin alignment in a tire lateral direction, a plurality of circumferentialgrooves, in a tread surface of the tread portion, disposed in alignmentin the tire lateral direction extending in the tire circumferentialdirection, including a central circumferential groove disposed on thetire equatorial plane, outer circumferential grooves disposed outermostin the tire lateral direction, and intermediate circumferential groovesdisposed between the central circumferential groove and the outercircumferential grooves, a land portion disposed between the centralcircumferential groove and one of the outer circumferential groovesbeing divided into at least three in the tire lateral direction by theintermediate circumferential grooves; and a plurality of lateralgrooves, in each of the land portions, disposed in alignment in the tirelateral direction at an incline with respect to the tire circumferentialdirection, opening at both ends to the circumferential grooves adjacentin the tire lateral direction, wherein an inclination angle of an acuteangle of the lateral grooves with respect to the tire circumferentialdirection is smallest in the land portion closest to the tire equatorialplane and is larger in the land portion as it goes to the tire lateraldirection outer side.

According to the pneumatic tire, by the inclination angles of the acuteangles of the lateral grooves with respect to the tire circumferentialdirection being configured as such, the rigidity of the land portionclosest to the tire equatorial plane is reduced, and the rigidity of theland portion is progressively higher going outward in the tire lateraldirection. Thus, the rigidity difference between the tire equatorsurface region and the outer region in the tire lateral direction causedby the circumferential belt is suppressed while obtaining the effect ofsuppressing the radial growth when the tire is new and thereafter byincreasing the circumferential rigidity by using the circumferentialbelt. As a result, the circumferential rigidity of the tread portionacross the tire lateral direction can be made uniform and uneven wearcan be suppressed, thus the improved uneven wear resistance performancecan be provided even with a configuration including the circumferentialbelt.

In the pneumatic tire according to an aspect of the present technology,preferably the lateral grooves between the two of the land portionsadjacent in the tire lateral direction have a difference in theinclination angle being larger as it goes closer to the tire equatorialplane and smaller as it goes closer to the tire lateral direction outerside.

According to the pneumatic tire, the difference between the inclinationangles of the acute angles of the lateral grooves with respect to thetire circumferential direction in the two land portions adjacent in thetire lateral direction corresponds to the difference in rigidity betweenthe two adjacent land portions. By the difference between theinclination angles of the acute angles of the lateral grooves withrespect to the tire circumferential direction being larger as it goescloser to the tire equatorial plane and smaller as it goes closer to thetire lateral direction outer side, the difference in rigidity betweenthe land portions adjacent in the tire lateral direction is largertowards the tire equatorial plane. This allows excessive circumferentialrigidity in the tire equatorial plane region caused by thecircumferential belt to be suppressed. As a result, the circumferentialrigidity of the tread portion across the tire lateral direction canfurther be made uniform and uneven wear can be suppressed, thus theeffect of significantly improving uneven wear resistance performance canbe provided even with a configuration including the circumferentialbelt.

In the pneumatic tire according to an aspect of the present technology,preferably a center region being a region defined between the outercircumferential grooves, a tire lateral direction dimension Wf of thecenter region and a tire lateral direction dimension Wg of thecircumferential belt satisfy a relationship Wg/Wf≥1.03.

In the region outward of the circumferential belt in the tire lateraldirection, circumferential rigidity is not high, thus, in this region,it is not necessary to make rigidity uniform by using the inclinationangles of the acute angles of the lateral grooves with respect to thetire circumferential direction. As a result, the center region ispreferably disposed within the range of the circumferential belt.

In the pneumatic tire according to an aspect of the present technology,preferably when the pneumatic tire is mounted on a regular rim, inflatedto a regular internal pressure, and in an unloaded state, a differencein tire radial direction dimension between both ends in the tire lateraldirection of the land portions is smaller as it goes closer to the tireequatorial plane and larger as it goes closer to the tire lateraldirection outer side, and a difference in tire radial directiondimension Do of the land portion located outermost in the tire lateraldirection and a difference in tire radial direction dimension Dm of theland portion adjacent thereto inward in the tire lateral directionsatisfy the relationship Do/Dm≥1.5.

According the pneumatic tire, the circumferential rigidity is smallerthe smaller the difference in tire radial direction dimension betweenboth ends in the tire lateral direction of the land portion, and on thecontrary the circumferential rigidity is larger the larger thedifference. Also, in relation to the difference in tire radial directiondimension Do of the land portion outermost in the tire lateral directionand the difference in tire radial direction dimension Dm of the landportion adjacent thereto inward in the tire lateral direction, thedifference in rigidity is larger the larger the difference in tireradial direction dimension Do located outward in the tire lateraldirection. Accordingly, as the circumferential rigidity is decreased inthe land portions against the increase of the circumferential rigidityin the tire equatorial plane region by the circumferential belt, thecircumferential rigidity is increased in the land portions against thedecrease of the circumferential rigidity in the region outward from thecircumferential belt, and the difference in rigidity of the landportions in the outer regions in the tire lateral direction arespecified by the relationship Do/Dm, a difference in circumferentialrigidity in the tire lateral direction in the land portions caused bythe circumferential belt can be suppressed. As a result, thecircumferential rigidity of the tread portion across the tire lateraldirection can further be made uniform and uneven wear can be suppressed,thus the effect of significantly improving uneven wear resistanceperformance can be provided even with a configuration including thecircumferential belt.

In the pneumatic tire according to an aspect of the present technology,preferably the land portion is formed to have a plurality of blocksdefined by the two circumferential grooves adjacent in the tire lateraldirection and the two lateral grooves adjacent in the tirecircumferential direction, the blocks are divided in the tire lateraldirection to form small blocks by a narrow groove that opens at bothends to two of the lateral grooves adjacent in the tire circumferentialdirection, and in the plurality of blocks, a surface area S_(I) of thesmall block closest to the tire equatorial plane and a surface areaS_(O) of the small block closest to the tire lateral direction outerside satisfy a relationship S_(O)/S_(I)≥1.01.

According to the pneumatic tire, in the blocks, by configuring thesurface area S_(O) of the small block located closest to the tirelateral direction outer side being greater than the surface area S_(I)of the small block closest to the tire equatorial plane, the rigidity onthe side closer to the tire lateral direction outer side in the blockcan be increased. As a result, the circumferential rigidity of the treadportion across the tire lateral direction can further be made uniformand uneven wear can be suppressed, thus the effect of significantlyimproving uneven wear resistance performance can be provided even with aconfiguration including the circumferential belt.

In the pneumatic tire according to an aspect of the present technology,preferably the land portion is formed to have a plurality of blocksdefined by the two circumferential grooves adjacent in the tire lateraldirection and the two lateral grooves adjacent in the tirecircumferential direction, and an aspect ratio of a tire circumferentialdirection dimension L and a tire lateral direction dimension We of eachof the blocks satisfies 1.2≤L/We≤2.0.

According to the pneumatic tire, the aspect ratio of the tirecircumferential direction dimension L of the block and the tire lateraldirection dimension We is configured to be in the range described aboveto make it easier for the block to have a difference in rigidity.

The present technology can provide improved uneven wear resistanceperformance while maintaining durability performance.

The present technology can provide improved uneven wear resistanceperformance even with a configuration including a circumferential belt.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a tread portion of a pneumatic tire accordingto a first embodiment of the present technology.

FIG. 2 is an enlarged view of the tread portion of the pneumatic tireaccording to a first embodiment of the present technology.

FIG. 3 is an enlarged view of another tread portion of the pneumatictire according to a first embodiment of the present technology.

FIG. 4 is an enlarged view of another tread portion of the pneumatictire according to a first embodiment of the present technology.

FIG. 5 is an enlarged cross-sectional view of the tread portion of thepneumatic tire according to a first embodiment of the presenttechnology.

FIG. 6 is an enlarged cross-sectional view of the tread portion of thepneumatic tire according to a first embodiment of the presenttechnology.

FIG. 7 is a table listing the results of performance tests of pneumatictires according to first examples of the present technology.

FIG. 8 is a table listing the results of performance tests of pneumatictires according to first examples of the present technology.

FIG. 9 is a meridian cross-sectional view of a portion of a pneumatictire according to a second embodiment of the present technology.

FIG. 10 is a plan view of a tread portion of the pneumatic tireaccording to a second embodiment of the present technology.

FIG. 11 is an enlarged plan view of the tread portion of the pneumatictire according to a second embodiment of the present technology.

FIG. 12 is an enlarged cross-sectional view of the tread portion of thepneumatic tire according to a second embodiment of the presenttechnology.

FIG. 13 is an enlarged cross-sectional view of the tread portion of thepneumatic tire according to a second embodiment of the presenttechnology.

FIG. 14 is an enlarged plan view of the tread portion of the pneumatictire according to a second embodiment of the present technology.

FIG. 15 is an enlarged meridian cross-sectional view of the pneumatictire according to a second embodiment of the present technology.

FIG. 16 is an enlarged plan view of the tread portion of the pneumatictire according to a second embodiment of the present technology.

FIG. 17 is an enlarged plan view of the tread portion of the pneumatictire according to a second embodiment of the present technology.

FIG. 18 is an enlarged plan view of the tread portion of the pneumatictire according to a second embodiment of the present technology.

FIG. 19 is a table listing the results of performance tests on pneumatictires according to second examples of the present technology.

FIG. 20 is a table listing the results of performance tests on pneumatictires according to second examples of the present technology.

DETAILED DESCRIPTION First Embodiment

A first embodiment of the present technology is described in detailbelow with reference to the drawings. However, the present technology isnot limited by the first embodiment. Constituents of the firstembodiment include elements that are essentially identical or that canbe substituted or easily conceived by a person skilled in the art.Furthermore, the modified examples described for the first embodimentcan be combined as desired within the scope apparent to those skilled inthe art.

FIG. 1 is a plan view of a tread portion of a pneumatic tire accordingto the first embodiment. FIG. 2 is an enlarged view of the tread portionof the pneumatic tire according to the first embodiment. FIG. 3 is anenlarged view of another tread portion of the pneumatic tire accordingto the first embodiment. FIG. 4 is an enlarged view of another treadportion of the pneumatic tire according to the first embodiment. FIG. 5is an enlarged cross-sectional view of the tread portion of thepneumatic tire according to the first embodiment. FIG. 6 is an enlargedcross-sectional view of the tread portion of the pneumatic tireaccording to the first embodiment.

Hereinafter, “tire circumferential direction” refers to thecircumferential direction with the rotation axis (not illustrated) of apneumatic tire 1 as the center axis. Additionally, “tire lateraldirection” refers to the direction parallel with the rotation axis.“Inward in the tire lateral direction” refers to the direction toward atire equatorial plane (not illustrated) in the tire lateral direction.“Outward in the tire lateral direction” refers to the direction awayfrom the tire equatorial plane in the tire lateral direction. “Tireradial direction” refers to the direction orthogonal to the rotationaxis. “Inward in the tire radial direction” refers to the directiontoward the rotation axis in the tire radial direction. “Outward in thetire radial direction” refers to the direction away from the rotationaxis in the tire radial direction. “Tire equatorial plane” is the planeorthogonal to the rotation axis that passes through the center of thetire width of the pneumatic tire 1.

As illustrated in FIG. 1, the pneumatic tire 1 of the first embodimentincludes a tread portion 2. The tread portion 2 is made of a rubbermaterial and exposed on the outermost side of the pneumatic tire 1 inthe tire radial direction, with the surface thereof, i.e., a treadsurface 2A, constituting the profile of the pneumatic tire 1.

The tread surface 2A of the tread portion 2 includes a plurality ofcircumferential grooves 3 (seven in the first embodiment illustrated inFIG. 1) extending in the tire circumferential direction and disposed inalignment in the tire lateral direction. Circumferential groove 3 refersto a groove having, for example, a groove width (Wa in FIGS. 2 and 5)from 8 mm to 15 mm and a groove depth (dimension from the openingposition on the tread surface 2A to the groove bottom, Ha in FIGS. 5 and6) from 10 mm to 28 mm.

Additionally, the tread portion 2 includes, in the tread surface 2A,ribs extending in the tire circumferential direction defined by adjacentcircumferential grooves 3. The tread portion 2 includes, in the treadsurface 2A, a plurality of lateral grooves 4 disposed in alignment inthe tire circumferential direction, extending in the tire lateraldirection and crossing the tire circumferential direction. The lateralgrooves 4 communicate with the circumferential grooves 3 at both ends.Accordingly, each rib defined by the circumferential grooves 3 isdivided by the lateral grooves 4 to form block-shaped land portions 5defined between the circumferential grooves 3. Lateral groove 4 refersto a groove having, for example, a groove width (Wb in FIG. 2) from 1 mmto 4 mm and a groove depth (dimension from the opening position on thetread surface 2A to the groove bottom, Hb in FIG. 6) from 1 mm to 5 mm.

Additionally, the tread portion 2 includes, in each land portion 5 ofthe tread surface 2A defined by the circumferential grooves 3 and thelateral grooves 4, two independent narrow grooves 6 extending in thetire circumferential direction disposed in alignment without meeting inthe tire lateral direction. The narrow grooves 6 communicate with thelateral grooves 4 at both ends. Accordingly, each land portion 5 definedby the circumferential grooves 3 and the lateral grooves 4 is dividedplurally in the tire lateral direction by the narrow grooves 6. Thus, acentral small land portion 5A and two side small land portions 5B areformed in each land portion 5. The central small land portion 5A isdefined by the lateral grooves 4 and the narrow grooves 6, and the sidesmall land portions 5B are defined by the circumferential groove 3, thelateral grooves 4, and the narrow groove 6. Narrow groove 6 refers to agroove having, for example, a groove width (Wc in FIGS. 2 and 5) from 1mm to 4 mm and a groove depth (dimension from the opening position onthe tread surface 2A to the groove bottom, Hc in FIG. 5) from 1 mm to 5mm.

The groove width Wc of the two narrow grooves 6 in each of the landportions 5 is less than the groove width Wa of the circumferentialgrooves 3. A bent portion 6A is formed in an intermediate portion of thenarrow grooves 6. The bent portion 6A is disposed inward in the tirelateral direction in the direction the two narrow grooves 6 oppose oneanother in the tire lateral direction from an imaginary straight line Aconnecting the ends of the narrow groove 6. Furthermore, the narrowgrooves 6 are disposed with bend points 6Aa of the bent portions 6Alocated offset from each other in the tire circumferential direction.

As illustrated in FIG. 2, in a configuration in which each narrow groove6 is formed with the bent portion 6A and bends once, the bend points 6Aaare the single bending point located inward in the tire lateraldirection in the direction the two narrow grooves 6 oppose one anotherin the tire lateral direction.

As illustrated in FIG. 3, in a configuration in which each narrow groove6 is formed with a plurality of the bent portions 6A and bends aplurality of times, the bend point 6Aa is the bending point locatedfurthest inward in the tire lateral direction in the direction the twonarrow grooves 6 oppose one another in the tire lateral direction(inward from a reference straight line B in the direction the two narrowgrooves 6 oppose one another).

As illustrated in FIG. 4, in a configuration in which each narrow groove6 is formed with a plurality of the bent portions 6A and bends aplurality of times and a plurality of bend points are located furthestinward in the tire lateral direction in the direction the two narrowgrooves 6 oppose one another in the tire lateral direction, the bendpoint 6Aa is a center point of a reference straight line C that connectsthe bending points located furthest inward at each end in the tirecircumferential direction. In such a configuration, as illustrated inFIG. 4, the bend point 6Aa may not correspond to a bending point.

According to the pneumatic tire 1 of the first embodiment with such aconfiguration, each of the land portions 5 defined by thecircumferential grooves 3 and the lateral grooves 4 are divided by thetwo narrow grooves 6 to form the central small land portion 5A and thetwo side small land portions 5B. This results in lower rigidity, whichallows the ground contact pressure when the tread portion 2 comes intocontact with the ground to be alleviated. Additionally, the length ofeach of the narrow grooves 6 is increased by having the bent portions 6Aformed in the narrow grooves 6. This further results in lower rigidity,which allows the ground contact pressure to be further alleviated. Thus,uneven wear resistance performance can be improved.

Furthermore, according to the pneumatic tire 1 of the first embodiment,the bent portion 6A is disposed inward in the tire lateral direction inthe direction the two narrow grooves 6 oppose one another from theimaginary straight line A connecting the ends of the narrow groove 6. Asa result, a reduction in the area of the side small land portions 5B onthe tread surface 2A is suppressed, and a decrease in rigidity isprevented. Moreover, the bend points 6Aa of the two narrow grooves 6 arelocated offset from each other in the tire circumferential direction sothat localized narrowing of the central small land portion 5A issuppressed, and thus, a decrease in rigidity is prevented. Thus,separation and chipping of the land portion 5 can be suppressed.

As a result, according to the pneumatic tire 1 of the first embodiment,uneven wear resistance performance can be improved while maintainingdurability performance by suppressing separation and chipping.

In the pneumatic tire 1 of the first embodiment, a bend angle α of thebent portion 6A of the narrow groove 6 preferably ranges from 90° to160°. As illustrated in FIGS. 2 to 4, the bend angle α is the smallerangle of the bend at the bent portion 6A.

According to the pneumatic tire 1, when the bend angle α of the bentportion 6A is 90° or greater, the bend becomes less sharp. As a result,separation and chipping is less likely to occur and the effect ofmaintaining durability is great. When the bending angle α of the bentportion 6A is 160° or less, the length of the narrow grooves 6 isincreased, and the effect of reducing rigidity is great.

As illustrated in FIG. 2, in the pneumatic tire 1 of the firstembodiment, 0.1≤Lc/L is preferably satisfied, where Lc is the shiftwidth of the bend points 6Aa of the two narrow grooves 6 in the tirecircumferential direction and L is the tire circumferential directiondimension of the land portion 5 in which the narrow grooves 6 areformed. The maximum value of the shift width Lc is within a range of theimaginary straight line A or less.

According to the pneumatic tire 1, by satisfying 0.1≤Lc/L, localizednarrowing of the central small land portion 5A between the two narrowgrooves 6 is further suppressed, and a decrease in rigidity can befurther prevented. As a result, the effect of maintaining durabilityperformance is great.

Additionally, according to the pneumatic tire 1 of the first embodiment,the lateral grooves 4 are formed extending at an incline with respect tothe tire lateral direction. An angle β with respect to the tire lateraldirection preferably ranges from 5° to 50°.

According to the pneumatic tire 1, when the angle β of the lateralgrooves 4 with respect to the tire lateral direction is greater than 5°,the length of the lateral grooves 4 is increased and the effect ofreducing rigidity is great. When the angle β of the lateral grooves 4with respect to the tire lateral direction is 50° or less, the angle isprevented from being sharp. As a result, separation and chipping is lesslikely to occur and the effect of maintaining durability is great.

Note that in the pneumatic tire 1 of the first embodiment, the lateralgrooves 4, in a rib defined by adjacent circumferential grooves 3, areinclined in the same direction with respect to the tire lateraldirection. Additionally, in the pneumatic tire 1 of the firstembodiment, the lateral grooves 4, in adjacent ribs, are also inclinedin the same direction with respect to the tire lateral direction. Insuch a configuration, each of the land portions 5 are uniform in shape,which contributes to improving uneven wear resistance performance.

In the pneumatic tire 1 of the first embodiment, the relationshipsHa>Hb, and Ha>Hc are preferably satisfied, where Ha is the groove depthof the circumferential grooves 3, Hb is the groove depth of the lateralgrooves 4, and Hc is the groove depth of the narrow grooves 6.

According to the pneumatic tire 1, the groove depth Hb of the lateralgrooves 4 and the groove depth Hc of the narrow grooves 6 are less thanthe groove depth Ha of the circumferential grooves 3 so that a decreasein rigidity of the land portion 5 in the tire circumferential directionis suppressed. Thus, the effect of maintaining durability is great.

In the pneumatic tire 1 of the first embodiment, Hb is preferably from 1mm to 5 mm and Hc is preferably from 1 mm to 5 mm, where Hb is thegroove depth of the lateral grooves 4 and Hc is the groove depth of thenarrow grooves 6.

According to the pneumatic tire 1, the groove depth Hb of the lateralgrooves 4 and the groove depth Hc of the narrow grooves 6 are configuredto be from 1 mm to 5 mm to suppress a decrease in rigidity of the landportions 5. As a result, the effect of maintaining durability is great.

In the pneumatic tire 1 of the first embodiment, the lateral grooves 4extend at an incline with respect to the tire lateral direction, and achamfer 4A is preferably formed in the corner portion of the landportion 5 where an acute angle with respect to the tire circumferentialdirection is formed.

According to the pneumatic tire 1, by providing the chamfer 4A,separation and chipping are less likely to occur and the effect ofmaintaining durability is great.

As illustrated in FIGS. 2 to 5, the pneumatic tire 1 of the firstembodiment preferably further includes a sipe 7 in the tread surface 2A.The sipe 7 communicate with the circumferential groove 3 at one end andterminate within the land portion 5 at the other end without meeting thenarrow groove 6. For the sipe 7, the relationships 0.3 mm≤Wd≤2.0 mm,0.3≤Hd/Ha≤1.0, and 0.03≤Ld/We≤0.2 are preferably satisfied, where Wd isthe groove width, Hd is the groove depth, Ld is the groove length, We isthe tire lateral direction dimension of the land portion 5, and Ha isthe groove depth of the circumferential groove 3.

According to the pneumatic tire 1, the sipe 7 reduce the rigidity of theland portion 5 so that ground contact pressure is alleviated. As aresult, the effect of improving uneven wear resistance performance isgreat.

First Examples

For the first examples, performance tests for uneven wear resistanceperformance and durability performance were performed on a plurality oftypes of test tires of different conditions (see FIGS. 7 and 8).

In the performance tests, pneumatic tires (heavy duty pneumatic tires)having a tire size of 445/50R22.5 were mounted on regular rims, inflatedto the regular internal pressure, and mounted on the trailer axle of atest vehicle (2-D·D vehicle (i.e., three axle vehicle with single tiresteering axle and tandem dual tire drive axle).

Here, “regular rim” refers to a “standard rim” defined by the JapanAutomobile Tyre Manufacturers Association Inc. (JATMA), a “Design Rim”defined by the Tire and Rim Association, Inc. (TRA), or a “MeasuringRim” defined by the European Tyre and Rim Technical Organisation(ETRTO). “Regular internal pressure” refers to a “maximum air pressure”defined by JATMA, the maximum value in “TIRE LOAD LIMITS AT VARIOUS COLDINFLATION PRESSURES” defined by TRA, or “INFLATION PRESSURES” defined byETRTO.

In the performance test for uneven wear resistance performance, the testvehicle was driven for 100000 miles (about 160000 km) and, thereafter,the area and depth of uneven wear that occurred in the land portionswere measured. The measurement results are expressed as index values andevaluated with the Conventional Example being assigned as the reference(100). In this evaluation, larger values are preferable as they indicateexcellent uneven wear resistance performance.

In the performance test for durability performance, the test vehicle wasdriven for 100000 miles (about 160000 km) and, thereafter, the number ofseparations and chipping that occurred in the land portions wasmeasured. The measurement results are expressed as index values andevaluated with the Conventional Example being assigned as the reference(100). In this evaluation, larger values are preferable as they indicatea lower number of separations and chipping and excellent durabilityperformance.

The pneumatic tire of the Conventional Example indicated in FIG. 7includes no narrow grooves. The pneumatic tires of each of theComparative Examples indicated in FIG. 7 include narrow grooves, but theconfiguration of the narrow grooves is different from the specificationsof the First Examples. On the other hand, the pneumatic tires of theExamples indicated in FIGS. 7 and 8 include narrow grooves with aconfiguration within the specifications.

As can be seen from the test results in FIGS. 7 and 8, the pneumatictires of Examples 1 to 12 have enhanced uneven wear resistanceperformance while maintaining durability performance.

Second Embodiment

A second embodiment of the present technology is described in detailbelow with reference to the drawings. However, the present technology isnot limited by the second embodiment. Constituents of the secondembodiment include elements that are essentially identical or that canbe substituted or easily conceived by a person skilled in the art.Furthermore, the modified examples described in the second embodimentcan be combined as desired within the scope apparent to one skilled inthe art.

FIG. 9 is a meridian cross-sectional view of a portion of a pneumatictire according to the second embodiment. FIG. 10 is a plan view of atread portion of the pneumatic tire according to the second embodiment.FIG. 11 is an enlarged plan view of the tread portion of the pneumatictire according to the second embodiment. FIGS. 12 and 13 are enlargedcross-sectional views of the tread portion of the pneumatic tireaccording to the second embodiment.

Hereinafter, “tire radial direction” refers to the direction orthogonalto the rotation axis (not illustrated) of a pneumatic tire 101. “Inwardin the tire radial direction” refers to the direction toward therotation axis in the tire radial direction. “Outward in the tire radialdirection” refers to the direction away from the rotation axis in thetire radial direction. “Tire circumferential direction” refers to thecircumferential direction with the rotation axis as the center axis.Additionally, “tire lateral direction” refers to the direction parallelwith the rotation axis. “Inward in the tire lateral direction” refers tothe direction toward a tire equatorial plane CL (tire equator line) inthe tire lateral direction. “Outward in the tire lateral direction”refers to the direction away from the tire equatorial plane CL in thetire lateral direction. “Tire equatorial plane CL” refers to the planeorthogonal to the rotation axis of the pneumatic tire 101 that passesthrough the center of the tire width of the pneumatic tire 101. “Tireequator line” refers to the line in the tire circumferential directionof the pneumatic tire 101 that lies on the tire equatorial plane CL. Inthe second embodiment, the tire equator line and the tire equatorialplane are denoted by the same reference sign CL.

A pneumatic tire 101 according to the second embodiment is a heavy dutypneumatic tire applied to a truck, a bus or the like. As illustrated inFIG. 9, the pneumatic tire 101 includes a tread portion 121, shoulderportions 122 located on both outward side thereof in the tire lateraldirection, and a sidewall portion and a bead portion continuing on fromeach of the shoulder portions 122 in that order. Note that in FIG. 9,the sidewall portion and the bead portion are omitted. The pneumatictire 101 also includes a carcass layer 124 and a belt layer 125.

The tread portion 121 is made of a rubber material (tread rubber) and isexposed on the outermost side of the pneumatic tire 101 in the tireradial direction, with the surface thereof constituting the profile ofthe pneumatic tire 101. A tread surface 121A is formed on the outercircumferential surface of the tread portion 121, in other words, on theroad contact surface that comes into contact with the road surface whenrunning.

The shoulder portions 122 are portions of the tread portion 121 locatedoutward in the tire lateral direction on both sides. Additionally, thesidewall portions, though not illustrated in the drawings, are exposedon the outermost sides of the pneumatic tire 101 in the tire lateraldirection. Additionally, the bead portion, though not illustrated in thedrawings, includes a bead core and a bead filler. The bead core isformed by winding a bead wire, which is a steel wire, into an annularshape. The bead filler is a rubber material that is disposed in thespace formed by an end of the carcass layer 124 in the tire lateraldirection being folded back at the position of the bead core.

Each of the end portions of the carcass layer 124 in the tire lateraldirection is folded back at the pair of bead cores from inward tooutward in the tire lateral direction, and the carcass layer 124 isstretched in a toroidal shape in the tire circumferential direction toform the framework of the tire. The carcass layer 124 is made of aplurality of carcass cords (not illustrated) disposed in alignment withan angle with respect to the tire circumferential direction along thetire meridian direction at an angle with respect to the tirecircumferential direction and coated with coating rubber. The carcasscords are made of steel or organic fibers (polyester, rayon, nylon, orthe like).

The belt layer 125 has a multilayer structure in which, in the secondembodiment, four belts 125A, 125B, 125C, 125D are layered in the tireradial direction. In the tread portion 121, the belt layer 125 isdisposed outward of the carcass layer 124 in the tire radial direction,i.e. on the outer circumference thereof, and covers the carcass layer124 in the tire circumferential direction. The belts 125A, 125B, 125C,125D are made of cords (not illustrated) disposed in a predeterminedangle with respect to the tire circumferential direction (for example,from 45° to 90° with respect to the tire circumferential direction) andcoated with coating rubber. The cords are made of steel or organicfibers (polyester, rayon, nylon, or the like). Each of the belts 125A,125B, 125C, 125D are disposed with the cords of the different layers inthe tire radial direction arranged in a criss-cross manner. Note thatfor the belt layer 125, it is sufficient that the cords of at least twobelts layered in the tire radial direction are arranged in a criss-crossmanner.

The belt layer 125 includes a circumferential belt 126. Thecircumferential belt 126 is made of cords (not illustrated) disposed inalignment in the tire lateral direction at an angle with respect to thetire circumferential direction of 0° (including)±5° and coated withcoating rubber. The cords are made of steel or organic fibers(polyester, rayon, nylon, or the like). The circumferential belt 126 isdisposed between the belts of the belt layer 125 at a position in thetire lateral direction encompassing the tire equatorial plane CL. In thesecond embodiment, the circumferential belt 126 is disposed between thebelts 125B, 125C. In other words, the circumferential belt 126 isdisposed underlaying or overlaying with respect to the tire radialdirection the two belts of the belt layer 125 with cords arranged in acriss-cross manner.

As illustrated in FIGS. 10 to 13, the tread surface 121 is provided withan odd number equal to or greater than seven (seven in the secondembodiment) of circumferential grooves 103 in the tread surface 121A.The circumferential grooves 103 are straight grooves extending in thetire circumferential direction parallel with the tire equator line CL.The circumferential grooves 103 are grooves with a groove width (Wa inFIGS. 11 and 13) being from 8 mm to 15 mm and a groove depth (dimensionfrom the opening position on the tread surface 121 a to the groovebottom, Ha in FIGS. 12 and 13) being from 10 mm to 28 mm. The treadsurface 121A is provided with an even number equal to or greater thaneight (eight in the second embodiment) rib-like land portions 105 inalignment each other defined by the circumferential grooves 103. Theland portions 105 extend in the tire circumferential direction.

The circumferential grooves 103 include a central circumferential groove103A disposed with its center on the tire equatorial plane CL, outercircumferential grooves 103C disposed outermost in the tire lateraldirection, and intermediate circumferential grooves 103B disposedbetween the central circumferential groove 103A and the outercircumferential grooves 103C. The land portions 105 includes inner landportions 105A disposed inward of both outer circumferential grooves 103Cin the tire lateral direction, and outer land portions 105B disposedoutward of both outer circumferential grooves 103C in the tire lateraldirection. Additionally, the region inward of both outer circumferentialgrooves 103C in the tire lateral direction where the inner land portions105A are disposed is referred to as center region Ce. That is, the innerland portions 105A are disposed in the center region Ce.

As illustrated in FIGS. 10 to 13, in the tread surface 121, each innerland portion 105A in the center region Ce of the tread portion 121A isprovided with lateral grooves (may also be referred to as lateraldirection grooves) 104. The lateral grooves 104 communicate at both endswith two circumferential grooves 103 adjacent in the tire lateraldirection. The lateral groove 104 refers to a groove with a groove width(Wb in FIG. 11) being from 1 mm to 4 mm and a groove depth (dimensionfrom the opening position on the tread surface 121A to the groovebottom, Hb in FIG. 12) being from 1 mm to 5 mm. In the inner landportions 105A, each of the land portions 105 defined by adjacentcircumferential grooves 103 is divided into blocks 151 by the lateralgroove 104. Note that as illustrated in FIG. 10, the opening portions ofthe lateral grooves 104 to the circumferential grooves 103 face oneanother across the groove walls of the circumferential grooves 103 inthe tire lateral direction, such that the lateral grooves 104 aredisposed continuously in the tire lateral direction. However no suchlimitation is intended, and the lateral grooves 104 may not becontinuous in the tire lateral direction.

As illustrated in FIGS. 10 to 13, the blocks 151 defined in the treadportion 121 by the circumferential grooves 103 and the lateral grooves104 are provided with a narrow groove 106 that opens at both ends to twolateral grooves 104 adjacent in the tire circumferential direction. Thenarrow groove 106 refers to a groove with a groove width (Wc in FIGS. 11and 13) being from 1 mm to 4 mm and a groove depth (dimension from theopening position on the tread surface 121A to the groove bottom, Hc inFIG. 13) being from 1 mm to 5 mm. The blocks 151 are divided by thenarrow groove 106 in the tire lateral direction to form small blocks151A. As illustrated in FIGS. 10 to 13, one block 151 is provided withtwo narrow grooves 106 disposed in alignment in the tire lateraldirection. Thus, the small blocks 151A include a central small block151Aa located at a center of the small block 151A in the tire lateraldirection and two outer small blocks 151Ab located on either side ofcentral small block 151Aa in the tire lateral direction. Additionally, abent portion 106A is formed in an intermediate portion of the narrowgroove 106 illustrated in FIG. 11. However, the bent portion 106A maynot be formed, and the narrow groove 106 may extend in a linear manner(see FIG. 17). It is sufficient that at least one narrow groove 106 isprovided in each block 151 (see FIG. 18). In such a configuration, thesmall block 151A includes two outer small blocks 151Ab located on eitherside in the tire lateral direction and no central small block 151Aa ispresent.

As illustrated in FIGS. 10 to 13, the tread portion 121 includes, in thetread surface 121A, a sipe 107 that communicate with the circumferentialgroove 103 at one end and terminate within the land portion 105 (smallblock 151A (outer small block 151Ab)) at the other end without meetingthe narrow groove 106. The sipe 107 can decrease the rigidity of theland portion 105 (small block 151A (outer small block 151Ab)) andalleviate ground contact pressure. Thus, uneven wear resistanceperformance can be improved. The same number of sipes 107 are providedin each block 151, and all the sipes 107 have the same groove width Wd,groove depth Hd, and groove length Ld. Accordingly, the sipes 107 do notcause variation in rigidity across the land portions 105. The sipe 107is a groove having the groove width Wd, the groove depth Hd, and thegroove length Ld satisfy the relationships with the tire lateraldirection dimension We of the land portion 105 and the groove depth Haof the circumferential groove 103 of: 0.3 mm≤Wd≤2.0 mm, 0.3≤Hd/Ha≤1.0,and 0.03≤Ld/We≤0.2.

As illustrated in FIGS. 11 and 12, in the tread portion 121, a chamfer104A may be formed in the corner portion of the land portion 105 (block151), where the lateral groove 104 opens at an incline to thecircumferential groove 103, forming an acute angle. By providing thechamfer 104A, separation and chipping are less likely to occur in theland portion 105 (block 151) and durability can be maintained.

FIG. 14 is an enlarged plan view illustrating the tread portion of thepneumatic tire according to the second embodiment. FIG. 15 is anenlarged meridian cross-sectional view of the pneumatic tire accordingto the second embodiment. FIGS. 16 to 18 are enlarged plan views of thetread portion of the pneumatic tire according to the second embodiment.

As illustrated in FIGS. 11 and 14, in the pneumatic tire 101 accordingto the second embodiment, an inclination angle θ of the acute angle ofthe lateral groove 104 with respect to the tire circumferentialdirection is the smallest in the land portion 105 (inner land portion105A) closest to the tire equatorial plane CL and is larger the closerthe land portion 105 (inner land portion 105A) is located to the tirelateral direction outer side. As illustrated in FIG. 14, in the secondembodiment, three land portions 105 (inner land portions 105A) areprovided on one side located outward from the tire equatorial plane CL(central circumferential groove 103A) as a border in the tire lateraldirection. There are the acute angle of the lateral groove 104 withrespect to the tire circumferential direction in the land portion 105(inner land portion 105A) closest to the tire equatorial plane CL is aninclination angle θ1, the acute angle of the lateral groove 104 withrespect to the tire circumferential direction in the land portion 105(inner land portion 105A) in an intermediate position in the tirelateral direction is an inclination angle θ2, and the acute angle of thelateral groove 104 with respect to the tire circumferential direction inthe land portion 105 (inner land portion 105A) outermost in the tirelateral direction is an inclination angle θ3 in the three land portions105 (inner land portions 105A). The inclination angles θ1, θ2, θ3 have arelationship satisfying θ1<θ2<θ3.

According to the pneumatic tire 101, by using the inclination angles θ1,θ2, θ3 of the acute angles of the lateral grooves 104 with respect tothe tire circumferential direction, the rigidity of the land portion 105(inner land portion 105A) closest to the tire equatorial plane CL isreduced, and the rigidity of the land portions 105 (inner land portions105A) is progressively higher going outward in the tire lateraldirection. Thus, the rigidity difference between the tire equatorsurface CL region and the outer region in the tire lateral directioncaused by the circumferential belt 126 is suppressed while obtaining theeffect of suppressing the radial growth when the tire is new andthereafter by increasing the circumferential rigidity by using thecircumferential belt 126. As a result, the circumferential rigidity ofthe tread portion 121 across the tire lateral direction can be madeuniform and uneven wear can be suppressed, thus the improved uneven wearresistance performance can be provided even with a configurationincluding the circumferential belt 126.

Note that the inclination angle θ of the acute angle of the lateralgroove 104 with respect to the tire circumferential direction ispreferably from 50° to 80°. When the inclination angle θ of the acuteangle of the lateral groove 104 with respect to the tire circumferentialdirection approaches 0°, the rigidity of the acute angle portion isweakened, which may cause separation and chipping. When the inclinationangle θ of the acute angle of the lateral groove 104 with respect to thetire circumferential direction approaches 90°, rigidity is excessivelyhigh and the difference in rigidity between the blocks 151 closer to thetire equatorial plane CL and the blocks 151 located outward in thelateral direction is difficult to differ. Thus, uneven wear resistanceperformance is reduced. Thus, the inclination angle θ of the acute angleof the lateral groove 104 with respect to the tire circumferentialdirection is preferably from 50° to 80° to suitably maintain rigidity inthe blocks 151. As illustrated in FIG. 10, the opening portions of thelateral grooves 104 to the circumferential grooves 103 face one anotheron each of the groove walls of the circumferential grooves 103 in thetire lateral direction. The inclination angles θ (θ1, θ2, θ3) startingfrom the tire equatorial plane CL going outward in the tire lateraldirection have the relationship described above. Thus, the lateralgrooves 104 are arranged in roughly an S-shape in the entire centerregion Ce from end to end in the tire lateral direction. Furthermore,the blocks 151, similar to the lateral grooves 104, are also arranged inroughly an S-shape in the entire center region Ce from end to end in thetire lateral direction. By arranging the lateral grooves 104 and theblocks 151 in roughly an S-shape continuous in the tire lateraldirection, uniform circumferential rigidity of the tread portion 121across the tire lateral direction can be easily achieved. Note thatuniform circumferential rigidity across the tire lateral direction canstill be achieved with the lateral grooves 104 and the blocks 151 notbeing continuous in the tire lateral direction.

In the pneumatic tire 101 of the second embodiment, the difference inthe inclination angle θ of the acute angle of the lateral groove 104with respect to the tire circumferential direction of two land portions105 (inner land portions 105A) adjacent in the tire lateral direction ispreferably larger as it goes closer to the tire equatorial plane CL andsmaller as it goes closer to the tire lateral direction outer side. Asillustrated in FIG. 14, in the second embodiment, three land portions105 (inner land portions 105A) are provided on one side located outwardfrom the tire equatorial plane CL (central circumferential groove 103A)as a border in the tire lateral direction. There are the acute angle ofthe lateral groove 104 with respect to the tire circumferentialdirection in the land portion 105 (inner land portion 105A) closest tothe tire equatorial plane CL is an inclination angle θ1, the acute angleof the lateral groove 104 with respect to the tire circumferentialdirection in the land portion 105 (inner land portion 105A) in anintermediate position in the tire lateral direction is an inclinationangle θ2, and the acute angle of the lateral groove 104 with respect tothe tire circumferential direction in the land portion 105 (inner landportion 105A) outermost in the tire lateral direction is an inclinationangle θ3 in the three land portions 105 (inner land portions 105A).Additionally, the differences between the inclination angles θ of theacute angles of the lateral grooves 104 with respect to the tirecircumferential direction in the two land portions 105 (inner landportions 105A) adjacent in the tire lateral direction are θ2-θ1 andθ3-θ2. Here, θ2-θ1 and θ3-θ2 satisfy the relationship θ2-θ1>θ3-θ2.

According to the pneumatic tire 101, the difference between theinclination angles θ of the acute angles of the lateral grooves 104 withrespect to the tire circumferential direction in the two land portions105 (inner land portions 105A) adjacent in the tire lateral directioncorresponds to the difference in rigidity between the two adjacent landportions 105 (inner land portions 105A). Accordingly, by the differencebetween the inclination angles θ of the acute angles of the lateralgrooves 104 with respect to the tire circumferential direction beinglarger as it goes closer to the tire equatorial plane CL and smaller asit goes closer to the tire lateral direction outer side, the differencein rigidity between the land portions 105 (inner land portions 105A)adjacent in the tire lateral direction is larger as it goes closer tothe tire equatorial plane CL. This allows excessive circumferentialrigidity in the tire equatorial plane CL region caused by thecircumferential belt 126 to be suppressed. As a result, thecircumferential rigidity of the tread portion 121 across the tirelateral direction can further be made uniform and uneven wear can besuppressed, thus the effect of significantly improving uneven wearresistance performance can be provided even with a configurationincluding the circumferential belt 126.

Note that θ2-θ1 and θ3-θ2 preferably satisfy the relationship1°≤(θ2-θ1)-(θ3-θ2)≤5°, so that the circumferential rigidity of the treadportion 121 across the tire lateral direction can be further madeuniform by the lateral grooves 104.

As illustrated in FIG. 9, in the pneumatic tire 101 of the secondembodiment, the region inward of both outer circumferential grooves103C, 103C in the tire lateral direction where the inner land portions105A are disposed is referred to the center region Ce, and a tirelateral direction dimension Wf of the center region Ce and a tirelateral direction dimension Wg of the circumferential belt 126preferably satisfy the relationship Wg/Wf≥1.03.

In the region outward of the circumferential belt 126 in the tirelateral direction, circumferential rigidity is not high, thus, in thisregion, it is not necessary to make rigidity uniform by using theinclination angles θ of the acute angles of the lateral grooves 104 withrespect to the tire circumferential direction. Accordingly, the centerregion Ce is preferably disposed within the range of the circumferentialbelt 126.

Note that the tire lateral direction dimensions Wf, Wg preferablysatisfy the relationship Wg/Wf≥1.05, so that the range in which rigidityis made uniform by using the inclination angles θ of the acute angles ofthe lateral grooves 104 with respect to the tire circumferentialdirection is sufficiently disposed within the range of thecircumferential belt 126.

In the pneumatic tire 101 of the second embodiment, when the pneumatictire 101 is mounted on a regular rim, inflated to the regular internalpressure, and in an unloaded state, the difference in tire radialdirection dimension between both ends in the tire lateral direction ofthe land portions 105 (inner land portions 105A) is preferably smalleras it goes closer to the tire equatorial plane CL and larger as it goescloser to the tire lateral direction outer side, and a difference intire radial direction dimension Do of the land portion 105 (inner landportion 105A) located outermost in the tire lateral direction and adifference in tire radial direction dimension Dm of the land portion 105(inner land portion 105A) adjacent thereto inward in the tire lateraldirection preferably satisfy the relationship Do/Dm≥1.5. As illustratedin FIG. 15, in the second embodiment, three land portions 105 (innerland portions 105A) are provided on one side located outward from thetire equatorial plane CL (central circumferential groove 103A) in thetire lateral direction. There are a difference in tire radial directiondimension between both ends in the tire lateral direction of the landportion 105 (inner land portion 105A) closest to the tire equatorialplane CL is D1, a difference in tire radial direction dimension betweenboth ends in the tire lateral direction of the land portion 105 (innerland portion 105A) in an intermediate position in the tire lateraldirection is D2, and a difference in tire radial direction dimensionbetween both ends in the tire lateral direction of the land portion 105(inner land portion 105A) outermost in the tire lateral direction is D3in the three land portions 105 (inner land portions 105A). Thedifferences in tire radial direction dimension D1, D2, D3 satisfy therelationship D1<D2<D3, and the difference in tire radial directiondimension Do (D3) of the land portion 105 (inner land portion 105A)outermost in the tire lateral direction and the difference in tireradial direction dimension Dm (D2) of the land portion 105 (inner landportion 105A) adjacent thereto inward in the tire lateral directionsatisfy the relationship D3/D2≥1.5.

Here, “regular rim” refers to a “standard rim” defined by the JapanAutomobile Tyre Manufacturers Association Inc. (JATMA), a “Design Rim”defined by the Tire and Rim Association, Inc. (TRA), or a “MeasuringRim” defined by the European Tyre and Rim Technical Organisation(ETRTO). “Regular internal pressure” refers to a “maximum air pressure”defined by JATMA, the maximum value in “TIRE LOAD LIMITS AT VARIOUS COLDINFLATION PRESSURES” defined by TRA, or “INFLATION PRESSURES” defined byETRTO.

According the pneumatic tire 101, the circumferential rigidity issmaller as the difference in tire radial direction dimension betweenboth ends in the tire lateral direction of the land portion 105 (innerland portion 105A) is smaller, and the circumferential rigidity islarger as the difference is lager. Also, in relation to the differencein tire radial direction dimension Do of the land portion 105 (innerland portion 105A) outermost in the tire lateral direction and thedifference in tire radial direction dimension Dm of the land portion 105(inner land portion 105A) adjacent thereto inward in the tire lateraldirection, the difference in rigidity is larger as the difference intire radial direction dimension Do located outward in the tire lateraldirection is larger. Accordingly, as the circumferential rigidity isdecreased in the land portions 105 (inner land portions 105A) againstthe increase of the circumferential rigidity in the tire equatorialplane CL region by the circumferential belt 126, the circumferentialrigidity is increased in the land portions 105 (inner land portions105A) against the decrease of the circumferential rigidity in the regionoutward from the circumferential belt 126, and the difference inrigidity of the land portions 105 in the outer regions in the tirelateral direction are specified by the relationship Do/Dm, a differencein circumferential rigidity in the tire lateral direction in the landportions 105 (inner land portions 105A) caused by the circumferentialbelt 126 can be suppressed. As a result, the circumferential rigidity ofthe tread portion 121 across the tire lateral direction can further bemade uniform and uneven wear can be suppressed, thus the effect ofsignificantly improving uneven wear resistance performance can beprovided even with a configuration including the circumferential belt126.

Note that the differences in tire radial direction dimension Do, Dmpreferably satisfy the relationship Do/Dm≥2.0. This allows thedifference in rigidity of the land portions 105 (inner land portions105A) in outer end regions of the circumferential belt 126 in the tirelateral direction, where the difference in circumferential rigidity islarge, to be increased, thus allowing the circumferential rigidity ofthe tread portion 121 in the tire lateral direction to be further madeuniform.

As illustrated in FIG. 11, in the pneumatic tire 101 of the secondembodiment, the land portion 105 (inner land portion 105A) is formedinto the block 151 defined by two circumferential grooves 103 adjacentin the tire lateral direction and two lateral grooves 104 adjacent inthe tire circumferential direction, and the block 151 is divided in thetire lateral direction by the narrow grooves 106 that open on both endsto the two lateral grooves 104 adjacent in the tire circumferentialdirection to form the small blocks 151A. As illustrated in FIGS. 16 to18, in the blocks 151, a surface area S_(I) of the outer small block151Ab closest to the tire equatorial plane CL and a surface area S_(O)of the outer small block 151Ab closest to the tire lateral directionouter side preferably have the relationship S_(O)/S_(O)≥1.01.

According to the pneumatic tire 101, in the blocks 151, by the surfacearea S_(O) of the outer small block 151Ab located closest to the tirelateral direction outer side being greater than the surface area S_(I)of the outer small block 151Ab closest to the tire equatorial plane CL,the rigidity on the side closer to the tire lateral direction outer sidein the block 151 can be increased. As a result, the circumferentialrigidity of the tread portion 121 across the tire lateral direction canfurther be made uniform and uneven wear can be suppressed, thus theeffect of significantly improving uneven wear resistance performance canbe provided even with a configuration including the circumferential belt126.

Note that the surface areas S_(I), S_(O) preferably satisfy therelationship 1.03≤S_(O)/S_(I)≤1.10. This ensures that the difference inrigidity in the blocks 151 in the tire lateral direction is notexcessive. Additionally, the surface areas S_(I), S_(O) are notincluding the sipes 107 described above. In other words, the sipes 107do not cause variation in rigidity. Thus, as described above, the samenumber of sipes 107 are provided in each block 151, and all the sipes107 have the same groove width Wd, groove depth Hd, and groove lengthLd.

Furthermore, as illustrated in FIG. 11, in the pneumatic tire 101 of thesecond embodiment, the land portions 105 (inner land portions 105A) areformed into the blocks 151 defined by two circumferential grooves 103adjacent in the tire lateral direction and two lateral grooves 104adjacent in the tire circumferential direction, and the aspect ratio ofthe tire circumferential direction dimension L and the tire lateraldirection dimension We of each block 151 is preferably 1.2≤L/We≤2.0.

According to the pneumatic tire 101, the aspect ratio of the tirecircumferential direction dimension L and the tire lateral directiondimension We of the block 151 is configured to the range described aboveto make it easier for the block 151 to have a difference in rigidity.

Note that the aspect ratio of the tire circumferential directiondimension L and the tire lateral direction dimension We of the block 151preferably satisfies the range 1.4≤L/We≤1.8. This ensures that thedifference in rigidity in the block 151 is not excessive.

Second Examples

For the second examples, performance tests for uneven wear resistanceperformance were performed on a plurality of types of test tires ofdifferent conditions (see FIGS. 19 and 20).

In the performance tests, pneumatic tires (heavy duty pneumatic tires)having a tire size of 445/50R22.5 were mounted on regular rims(22.5″×14.00″) specified by TRA, inflated to the regular internalpressure (830 kPa), and mounted on the trailer axle of a test vehicle(6×4 tractor-trailer).

In the performance test for uneven wear resistance performance, the testvehicle was driven for 10 km, and thereafter, the groove depth of theinner circumferential grooves and the outer circumferential grooves weremeasured. This difference is measured as the uneven wear amount. Themeasurement results are expressed as index values and evaluated with theConventional Example being assigned as the reference (100). In thisevaluation, larger values are preferable as they indicate excellentuneven wear resistance performance.

The pneumatic tires according to the Conventional Example and Examples101 to 118 indicated in FIGS. 19 and 20 include a circumferential belt,three land portions divided in the tire lateral direction byintermediate circumferential grooves disposed between a centralcircumferential groove and an outer circumferential groove, and aplurality of lateral grooves disposed in alignment in the tire lateraldirection in each of the land portions, the lateral grooves beinginclined with respect to the tire circumferential direction and openingat both ends to the circumferential grooves adjacent in the tire lateraldirection. In the pneumatic tire of the Conventional Example, theinclination angle θ1 of the acute angle of the lateral groove withrespect to the tire circumferential direction in the land portionclosest to the tire equatorial plane CL, the inclination angle θ2 of theacute angle of the lateral groove with respect to the tirecircumferential direction in the land portion in an intermediateposition in the tire lateral direction, and the inclination angle θ3 ofthe acute angle of the lateral groove with respect to the tirecircumferential direction in the land portion outermost in the tirelateral direction have the relationship θ1=θ2=θ3. On the other hand, inthe pneumatic tires of Examples 101 to 118, θ1<θ2<θ3 is satisfied. Thepneumatic tires of the Conventional Example and Examples 101 to 110include no narrow grooves. The pneumatic tires of Examples 111 to 118include narrow grooves and the relationship of the surface areas S_(O),S_(I) of the small blocks is specified.

As can be seen from the test results in FIGS. 19 and 20, the pneumatictires of Examples 101 to 118 have enhanced uneven wear resistanceperformance.

The invention claimed is:
 1. A pneumatic tire, comprising: twocircumferential grooves, in a tread surface of a tread portion, disposedin alignment in a tire lateral direction extending in a tirecircumferential direction; a plurality of lateral grooves, in the treadsurface, disposed in alignment in the tire circumferential directionextending to cross the tire circumferential direction, the plurality oflateral grooves communicating at both ends with both of thecircumferential grooves and defining a block-shaped land portion eachbetween the circumferential grooves; and two narrow grooves, in thetread surface of the land portion, disposed in alignment in the tirelateral direction extending in the tire circumferential direction, thenarrow grooves communicating at both ends with a respective lateralgroove of the plurality of lateral grooves and dividing, in the tirelateral direction, the land portion defined by each of thecircumferential grooves and each of the lateral grooves into a pluralityof small land portions; wherein the two narrow grooves are formed in theland portion having a smaller groove width than the circumferentialgrooves and each comprising a bent portion in an intermediate portion,the bent portion being disposed inward in the tire lateral direction ina direction the narrow grooves oppose one another from an imaginarystraight line connecting ends of the respective narrow grooves, and bendpoints of the bent portions being disposed at positions offset from oneanother in the tire circumferential direction an entirety of therespective narrow grooves lying inwards of the imaginary straight linein the direction the narrow grooves oppose one another.
 2. The pneumatictire according to claim 1, wherein the bent portions of the narrowgrooves have a bend angle ranging from 90° to 160°.
 3. The pneumatictire according to claim 1, wherein a relative shift width Lc in the tirecircumferential direction of each of the bend point of the two narrowgrooves and a tire circumferential direction dimension L of the landportion in which the narrow grooves are formed satisfy 0.1≤Lc/L.
 4. Thepneumatic tire according to claim 1, wherein the lateral groove isformed extending at an incline with respect to the tire lateraldirection, with an angle with respect to the tire lateral directionranging from 5° to 50°.
 5. The pneumatic tire according to claim 1,wherein relationships Ha>Hb, and Ha>Hc are satisfied, where Ha is agroove depth of the circumferential grooves, Hb is a groove depth of thelateral grooves, and Hc is a groove depth of the narrow grooves.
 6. Thepneumatic tire according to claim 1, wherein Hb ranges from 1 mm to 5 mmand Hc ranges from 1 mm to 5 mm, where Hb is a groove depth of thelateral grooves and Hc is a groove depth of the narrow grooves.
 7. Thepneumatic tire according to claim 1, wherein the lateral grooves areformed extending at an incline with respect to the tire lateraldirection; and a chamfer is formed in a corner portion of the landportion with an acute angle with respect to the tire circumferentialdirection.
 8. The pneumatic tire according to claim 1, furthercomprising a sipe, in the tread surface, communicating at one end withone of the circumferential grooves and terminating at another end withinthe land portion; and wherein relationships 0.3 mm≤Wd≤2.0 mm,0.3≤Hd/Ha≤1.0, and 0.03≤Ld/We≤0.2 are satisfied, where Wd is a groovewidth of the sipe, Hd is a groove depth of the sipe, Ld is a groovelength of the sipe, We is a tire lateral direction dimension of the landportion, and Ha is a groove depth of the circumferential grooves.
 9. Thepneumatic tire according to claim 1, wherein the two narrow grooves eachcomprise only a single bent portion in the intermediate portion.